Prolongation of the tool life has been practiced by depositing titanium carbide, titanium nitride, titanium carbonitride, Al.sub.2 O.sub.3, or another coating layer on the surface of a cemented-carbide cutting tool. Chemical vapor deposition (CVD), plasma CVD, and physical vapor deposition processes have been widely used for providing the coating layer.
However, the wear resistance of the coating layers has been insufficient, and the tool life has been shortened due to damage to or flaking of the coating layer when these coated cemented-carbide cutting tools are used particularly for the following machining: (1) machining, such as high-speed cutting of steel or high-speed machining of cast iron, that requires wear resistance and crater resistance in the coating layer at high temperatures, and (2) machining, such as small-parts machining, that has many machining processes and many leading parts on the workpiece.
In order to surmount these problems, controlling the structure and oriented texture of the coated layer has been studied on the multiple coated-layer structure in which the outer layer comprises Al.sub.2 O.sub.3 and the inner layer comprises titanium carbide or titanium carbonitride, for example, which is superior in hardness as well as in bonding with cemented carbides. For example, published Japanese patent application Tokuhyohei 9-507528 has disclosed a coating method in which Al.sub.2 O.sub.3 having an .alpha.-type crystal structure, which is stable at high temperatures, is given a certain amount of oriented texture in order to improve the high-temperature properties. Although the Al.sub.2 O.sub.3 having an .alpha.-type crystal structure is said to be superior in high-temperature properties, the material is well known to have difficulty in obtaining high bonding strength that prevents flaking at the time of cutting. In the above-mentioned prior technique also endeavor has been made to obtain high bonding strength by controlling the moisture content at the initial stage of the coating of Al.sub.2 O.sub.3. However, it cannot be said that sufficient bonding strength is obtained by this technique.